This invention relates to disproportionation/isomerization reactions, such as the Henkel reaction. More particularly, this invention relates to the disproportionation of the salt of a monocarboxylic acid, such as potassium naphthoate, to form the salt of a dicarboxylic acid, such as the salt of isomers of naphthalene dicarboxylic acid. Still more particularly, this invention relates to a combination of modifications of the elements of a disproportionation reaction that provides improvements in product yield of as much as 40%.
Aromatic dicarboxylic acids are highly useful organic compounds. They are often used as monomers for the preparation of polymeric materials. 2,6-naphthalene dicarboxylic acid (2,6-NDA) is a particularly useful aromatic carboxylic acid, because it can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate), PEN. Fibers and films manufactured from PEN display improved strength and superior thermal properties compared with other polyester materials such as polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cords, and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications.
It is known in the art to prepare aromatic dicarboxylic acids such as 2,6-NDA by primarily two methods. One is the liquid phase, metal catalyzed oxidation of an alkyl or acyl substituted aromatic compound. This method is described, for example, in U.S. Pat. Nos. 2,833,816; 3,856,855; 3,870,754; 4,933,491; and 4,950,786.
Alternatively, naphthalene monocarboxylic acid and naphthalene dicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can be converted to 2,6-NDA, using a disproportionation reaction in the case of the monocarboxylic acids, or a rearrangement reaction in the case of other naphthalene dicarboxylic acids. Henkel and Cie first patented a reaction of naphthoic acid salts to 2,6 NDA in the late 1950s. (See U.S. Pat. No. 2,823,231 and U.S. Pat. No. 2,849,482). In these references, the product yield of the disproportionation reaction is about 65% at best. Of course it would be desirable to improve on these yields.
Currently in the art 2,6-NDA for commercial use is prepared by oxidation, even though that route is plagued with difficulties. The most common process for making 2,6 NDA starts with relatively expensive o-xylene and butadiene feedstocks, as discussed, for example, in U.S. Pat. No. 5,510,563 and U.S. Pat. No. 5,329,058 and incurs substantial yield losses of these starting materials. Following the synthesis and purification of 2,6 dimethylnaphthalene (2,6 DMN), 2,6 DMN is oxidized to produce crude NDA product which forms as a solid with impurities trapped within. In order to remove these impurities to a sufficiently low level acceptable for polymerization, the 2,6 NDA product must be purified via multiple steps. These steps typically involve esterification, so that the end product is 2,6-naphthalene dicarboxylate, an ester, rather than the preferred 2,6 napthalene dicarboxylic acid. Esterification to naphthalene dicarboxylate (NDC) is necessary to eliminate the impurities, as discussed in U.S. Pat. No. 5,254,719 and 4,886,901.
Various improvements in a route to NDA based on disproportionation/rearrangement have been claimed in the art. In this work Henkel preparations which claimed to have improved yields depended on catalysts containing halogen-containing corrosive salts or other toxic and irritating materials. Other research that used zinc as a catalyst reported the making of zinc salts of the aromatic acids, a process involving added capital and difficulty in recycling the zinc. Yields of these processes were, even so, low due to failure to observe the effect of the critical components of the reaction and the effect of their ranges on the result of the reaction.
U.S. Pat. No. 2,919,273 claims improvements in yield in the thermal rearrangement of salts of cyclic carboxylic acids using as the catalyst salts of catalytically-active bivalent metals, wherein said metals are present in the form of compounds of the general formula selected from the group consisting of Alk2(MeX2Y2) and Alk(MeX2), where Alk is an alkali metal cation, Me is a bivalent catalytically active metal selected from the group consisting of cadmium, zinc and lead, and X is a halogen ion and Y is an ion selected from the group consisting of halogen and carbonate ions. This reference does not specifically address or have examples of disproportionation of potassium naphthoate.
The object of U.S. Pat. No. 3,641,130 is to obtain greater efficiency in a disproportionation reaction and claims an advantage carrying out said reaction in the presence of at least one adjuvant compound of the formula Rxe2x80x94Xxe2x80x94M, wherein R is an alkyl, cycloalkyl, aryl radical, or combination thereof having from 1 to 15 carbon atoms therein, X is oxygen or sulfur, and M is hydrogen or an alkali metal.
U.S. Pat. No. 3,751,456 claims an improvement in a disproportionation reaction using as a catalyst a mixture of cadmium iodide and sodium iodide.
U.S. Pat. No. 3,875,218 claims advantages using as a catalyst a metal salt of an aromatic carboxylic acid, said metal being selected from the group consisting of zinc, cadmium, mercury, lead, and iron. The examples demonstrate the use of zinc benzoate.
U.S. Pat. No. 4,820,868 claims an improvement in a process for the preparation of a naphthalene 2,6 -dicarboxylic acid dialkali metal salt comprising using naphthalene as a reaction medium, wherein the amount of naphthalene is 0.5 to 10 times the amount of the starting material by weight.
There is still a need in the art for greater conversion and yield in the production of 2,6-NDA, the preferred monomer for the production of polyethylenenaphthalate (PEN).
An object of the present invention is to improve rates of conversion to 2,6- naphthalene dicarboxylic acid (2,6-NDA). Another object is to increase the total yield of 2,6-NDA.
The present invention provides a method for increasing the disproportionation yields (Henkel II) of the dipotassium salts of 2,6-NDA from ca. 70% to as much as 95-100%, or more, resulting in a far more economical process. (Yields in excess of 100% basis the formal disproportionation reaction are possible provided some of the naphthoate salt starting material is carboxylated instead of disproportionated).
In accordance with the foregoing the present invention provides a process for disproportionation of potassium naphthoate to the dipotassium salt isomers of 2,6-NDA with reproducible improved yields which comprises reacting naphthoic acid in the presence of excess base to produce a feed comprising a finely dispersed disordered salt mixture of excess base salts and naphthoic acid salts, wherein the feed is prepared by the steps of:
a) Reacting naphthoic acid in the presence of at least 0.001 to 0.30 moles of excess base (excess beyond the amount necessary to form naphthoate salt of 1:1 acid to cation ratio) selected from the group consisting of carbonates and bicarbonate to form a solid salt;
b) Drying said salt mixture by a method which forms a highly mixed disordered salt mixture characterized by:
(1) A differential scanning calorimeter(DSC) signature characterized by low melting peaks not previously observed in potassium naphthoate salt; and/or
(2) A characteristic Xray diffraction pattern.
c) Disproportionating said solid salts in the presence of a disproportionation catalyst to form the dipotassium salts of 2,6-NDA.
When preparing feed salts for the Henkel disproportionation reaction in literature of the prior art, care is usually taken to prepare organic salts of precise stoichiometry. Raecke, (See U.S. Pat. No. 2,823,231) for example, back-titrated potassium acid salts with aqueous HCl to produce a solution of pH 6-7, resulting in a ratio of about 0.96(or less) to 1.00 potassium: aromatic acid. Using this sort of procedure yields are typically 60-70% of theoretical. In the present invention we have discovered process conditions that provide a reproducible yield of as high as 98% or more for naphthoic acid salt disproportionation to 2,6-NDA as opposed to published yields of about 65-75%.
Normally when employing a disproportionation reaction such as the Henkel, a significant yield loss occurs during the reaction. This loss, even in the best of circumstances, is usually 3% or more of the weight of the naphthalene dicarboxylic acids (NDAs) theoretically expected to be produced. This loss arises from a mixture of causes, such as coupling of aromatic radicals to form binaphthyls and higher condensed species, decarboxylation of naphthoic acids to naphthalene, and other undesired reactions. In the absence of charging other carboxylic acid salts (e.g. tricarboxylic benzene acids, or potassium formates, and the like) there is no precedent for obtaining a yield of NDA which exceeds the theoretical yield given by the equation for the Henkel II reaction:
2(potassium-2-naphthoic acid)xe2x86x921 naphthalene+1 NDA, where the NDA product is a mixture of isomers, but usually mostly 2,6-NDA.
In the present invention we have found the process elements that are the most critical in obtaining reproducible high yields. The inventive process consists of using an excess base, particularly an excess of the carbonate and bicarbonate salts formed by the CO2 precipitation of 2,6-NDA in the acid form. The process also involves preparation of a finely dispersed mixture of the excess base salts with the naphthoic acid salts which are characterized by observable low melting peaks in differential scanning calorimeter (DSC) scans of the feed salts. Further, the reaction may be accelerated by the addition of small amounts of larger alkali cation salts (e.g. Cs) which also seem to depress the melting point and improve the mixing of the carbonic and naphthoic salts. The key elements for producing higher yields of 2,6-NDA are:
1) Preparation of a disproportionation feed of finely dispersed salt materials containing a particular XRD diffraction pattern and differential scanning calorimeter (DSC) signature characterized by low melting peaks not usually observable in the naphthoic acid salt, prepared using 0.001 to 0.30 moles of excess base, preferably 0.03 to 0.30 moles of excess base, for each mole of naphthoic acid and preferably containing an effective amount of disproportionation catalyst, preferably a Group IIb metal salt, most preferably Zinc oxy salts; and
2) Strict regulation of the amount of water present, to ca. 1000 ppm, obtained by predrying the salt of naphthoic acid; and
In addition, the following elements of the process appear to contribute to the desired high yields:
3) The use of carbonate or bicarbonate, or mixtures thereof, as the base to form the potassium salts of naphthoic acid;
4) The use of zinc compound as the disproportionation catalyst;
5) Use of at least 100 psig of CO2 as a gas cap, preferably less than 500 psig, and most preferably about 250 psig;
6) Use of heat transfer media to provide a uniform vessel temperature for the reacting salt particles regulated to a constancy of  less than /=ca. 10xc2x0 C.; and
7) Optional use of liquid diluents such as naphthalene.
The starting material used in the present invention is a salt of an aromatic carboxylic acid, such as an alkali metal salt of benzoic acid, but particularly an alkali metal salt of naphthoic acid, a dialkali metal salt of naphthalene dicarboxylic acid, or a mixture thereof. As the alkali metal salt of naphthoic acid, there may be used a 1-isomer, a 2-isomer, and a mixture thereof. As the dialkali metal salt of naphthalene dicarboxylic acid, there may be used a 1,2-isomer, a 1,3-isomer, a 1,4-isomer, a 1,5-isomer, a 1,6-isomer, a 1,7-isomer, a 1,8-isomer, a 2,3-isomer, a 2,6-isomer, a 2,7-isomer, or a mixture thereof. The feed used to exemplify the invention was an alkali salt of naphthoic acid. The naphthoic acid was a combination of the 1-isomer and 2-isomer.
Suitable bases include alkali metal carbonates. The alkali metal can be selected from the group consisting of lithium, sodium, potassium, rubidium, and cesium, but is preferably potassium. Bases which can be used to provide the excess include, but are not limited to, K2CO3, KHCO3, Rb2CO3, RbHCO3, Cs2CO3, CsHCO3, and other strongly basic carbonates or bicarbonates. We have found it advantageous to use potassium carbonate or potassium bicarbonate. Potassium hydroxide will work, but for the purposes of the present invention, carbonates and bicarbonates are preferred.
Suitable temperatures for a disproportionation reaction are generally in the range of from about 340xc2x0 C. to 500xc2x0, and usually in the range of from about 400xc2x0 C. to 480xc2x0. The temperature range is dictated by specific salts and conditions and the selected temperature should be as uniform as possible. The preferred temperature in the present invention is from about 440xc2x0 C. to 460xc2x0 C. In the present invention, the use of heat transfer media assists in providing a uniform vessel temperature for the reacting salt particles with the objective of keeping the temperature within 10xc2x0 C. of the desired temperature of the disproportionation reactor.
Reacting naphthoic acid with excess base is a critical element of the present invention. An excess of base to naphthoic acid in the range of 1.01-2.0:1 is within the inventive concept, however greater than 2:1 is too high, because the organic part of the mixture will be too dilute. The benefits of the excess base are accomplished within the range of 1.03-1.8:1, preferably 1.1-1.6:1, moles potassium to moles of acid; and it is believed that, generally, about 1.15 to 1.3:1 moles of base to naphthoic acid will ensure reproducible improved yields. The optimum level of xe2x80x9coverbasingxe2x80x9d is between 0.001 and 0.3 moles of excess base per mole of naphthoic acid, although this is probably a function of the exact formulation and conditions used. Good results were obtained using 0.03 to 0.20 moles of excess base per mole of acid. It will be obvious to those skilled in the art that subsequent separation of the naphthalene dicarboxylic acid product will be more difficult, the more base is used.
The catalyst providing the best results in conjunction with the combination of conditions used for disproportionation in the present process is preferably a zinc compound. A number of zinc compounds would be suitable, including zinc halides such as zinc fluoride, zinc chloride, zinc bromide, and zinc iodide; zinc carboxylates such as zinc naphthoate and zinc naphthalene-dicarboxylate; zinc oxide, zinc carbonate; zinc sulfate and mixtures thereof. The zinc salts, preferred for cost and low toxicity, may be used as the oxide, carbonate, or other inorganic salt of the naphthoic acid feed, conveniently formed by reaction of the naphthoic acid with zinc oxide under elevated temperature. However, in the preferred embodiment the catalyst employed was ZnO.
A key part of the present invention is the preparation of disproportionation feed comprising a finely dispersed mixture of the excess base salts with the naphthoic acid salts. The excess base and potassium naphthoate can be mixed in water in almost any concentration, however greater than about 5% solids is preferred due to the lesser requirement for heating to remove the water and form the solid feed. The zinc catalyst as zinc oxide can be suspended as a solid in the mixture or it can be reacted with naphthoic acid to make zinc naphthoate, which can also be mixed in with the base and potassium naphthoate. It is noted, however, that the use of zinc oxide would dictate one set of drying conditions, while the use of zinc naphthoate would indicate other drying conditions, as would be apparent to one skilled in the art.
These salt mixtures, prepared as described, result in characteristic xray diffraction patterns which seem to be indicative of feeds which will provide higher yields. The powder xray diffraction patterns of the initial gently dried or air exposed feed salt mixture which will comprise the preferred feed is characterized by one or more of the following xe2x80x9ctwo thetaxe2x80x9d peaks (among others): 14.0, 28.5, 38.2, 13.6, 27.3, 32.0, and 36.7 degrees two theta. Not all the peaks need be in all samples, but in general feeds which have these peaks (as well as others) will give good yields. These peaks correspond to lattice spacings of 6.32, 3.13, 2.35, 6.52, 3.26, 2.80, and 2.45 Angstroms in Bragg d-spacing.
We have found a critical element in the process is maintaining the amount of water present in the disproportionation reactor to less than or about 1000 ppm. This is accomplished by drying the potassium naphthoate salt before disproportionation, xe2x80x9cpredryingxe2x80x9d as will be discussed below. Although too much water can present problems, it has been observed in the present invention that a small amount of water seems to actually be beneficial. It is speculated the beneficial effects are due to the effect of the water of introducing increased mobility into the salts, specifically allowing the salt complexes more rotational freedom and also by stabilizing the charged species formed as intermediates. It is also possible a small amount of water stabilizes the original finely divided mixed crystalline low melting materials that make the best feeds. However, a significant amount of water, say, for example in excess of ca. 700-1000 ppm, interferes with the reaction by beginning to favor the decomposition of the salts (decarboxylation). A significant amount of water, 0.2% or more, becomes damaging by promoting yield loss.
The goal of the drying step is to achieve a product salt that is a highly disordered salt mixture that is intimately mixed and to reduce any water present to less than 1000 ppm. Drying can be accomplished by heating the solids at about 150-200xc2x0 C., preferably about 175xc2x0 C. for about 1-3 hours, preferably about 2 hours under 0.8 torr mm Hg pressure. The Hg pressure in some instances might be higher, say as high as 2 torr, or higher. As mentioned above, drying can also be achieved by dripping the naphthoic acid potassium salt and carbonate aqueous mixture into hot oil. The mixture can also be spray dried, dried in a rotary evaporator or tumbler, or dried by some other method which quickly flashes water and forms a product with preferably small crystals.
It is desirable that the material be as intimately mixed as possible; the closer to molecular mixing, the better. The preferred salt feeds are also characterized by at least one or more low temperature DSC melt peaks not associated with pure potassium naphthoate or carbonate. There will be observable peaks in the range of about 2600 to 350xc2x0 or so, well below the expected peaks around 400xc2x0 C.
Thermogravimetric Analysis (TGA) of preferred feeds will reveal a relatively low onset temperature of non-drying weight loss as described in Example 8 and in copending U.S. Patent Application Ser. No. 60/151,604, incorporated herein by reference in the entirety.
A convenient way of forming the preferred disordered salt mixture is to charge a mixture of the organic naphthoic acid potassium (or mixed alkali) salt in water with dissolved inorganic (carbonate, bicarbonate) salts and suspended zinc oxide into a vessel of hot oil, preferably naphthalene or another stable hydrocarbon consistent with the process. Hot naphthalene, which has a boiling point 100-200xc2x0 C. above the salts, flashes water, and produces a porous and finely dispersed product. By this means the solution is rapidly converted to a porous solid of comparatively fine crystallite size and intimately mixed organic and inorganic salts. In order to analyze precisely for the amount of products produced, three repeated extractions of the salt phase with KOH in D2O and a suitable protonic internal standard for nmr (TSP, trimethyl sillyl sodium propionate) were made to analyze for the acids by quantitative nuclear magnetic resonance, and the hydrocarbon (naphthalene) portion of the product was digested in d6 DMSO (deuterio dimethyl sulfoxide) with trioxane as the internal protonic standard. By using test synthetic salt mixtures of K2-2,3 NDA, K2-2,6 NDA, K-2-NA, and naphthalene, it was shown that the error in the analysis by this method was less than 1% mole of the contained species analyzed.
A eutectic mixture can optionally be employed in the present invention. A eutectic mixture provides the lowest melting point of a mixture of two or more alkali metals that is obtainable by varying the percentage of the components. Eutectic mixtures have a definite minimum melting point compared with other combinations of the same metals. For example, though the melting point of Li2CO3 is 622xc2x0 C., in a eutectic mixture of alkali carbonates the melting point can be 400xc2x0 C. What is required, where a eutectic mixture is employed, is the right mixture of alkali metal carbonates where the melting point is less than about 400xc2x0 C. Generally the ratio of alkali metal carbonates in the eutectic mixture is about 1:1:1, but it can vary. One eutectic mixture used as a solvent was K2CO3, Rb2CO3, CS2CO3, and optionally Na2CO3.
In most disproportionation reactions one can observe in the art, suitable CO2 pressures are from about 200 to 10,000 psig. A more preferred range is generally from about 350 to 1100 psig. We have discovered that higher pressures will work, but there may be disadvantages with respect to costs and other factors. In combination with the other specified conditions of this process, a lower pressure worked very satisfactorily in contributing to better yields. A suitable CO2 pressure should be above 100 psig and good results were achieved using no higher than 500 psig. In the examples the preferred range is about 225-275 psig.
The reaction medium for the disproportionation reaction of the present invention is optionally and suitably naphthalene, but it can be any compound with sufficient thermal stability. It is not restricted to aromatic compounds, however aromatic compounds are suitable. Examples of suitable solvents include a single compound or mixture of compounds selected from a variety of aprotic polycyclic aromatic compounds, such as, for example, naphthalene, methylnaphthalene, dimethylnaphthalene, diphenyl ether, dinaphthyl ether, terphenyl, anthracene, phenanethrene, and mixtures thereof.
In the present invention a slurry containing the solid particulate salts and ZnO catalyst is fed to the disproportionation reactor at a temperature of about 450xc2x0 C. and about 250 psi, CO2 headspace, where KNA disproportionates to the dipotasium salt of 2,6 NDA and naphthalene. The reaction time can be up to three hours. The optimum residence time is about 1 to 1xc2xd hours.